South Korean regulators launched Monday an inquiry into the country's biggest banking network, after a system glitch suspected to be caused by a cyberattack left customers without access to services for several days last week.

News agency AFP reported that the country's Financial Supervisory Service and central bank officials visited today the Seoul headquarters of the National Agricultural Cooperative Federation, or Nonghyup, to investigate whether it had complied with computer security rules.

The trouble at Nonghyup began on Apr. 12, when the bank suffered a system crash that left customers unable to withdraw and transfer money, use their credit cards and take out loans. The bank's services were partially restored after three days, but some services remained unavailable as of Monday, said AFP.

According to Yonhap News Agency, online transactions were suspended for six days and the bank is working to fully restore services by Friday.

Quoting Nonghyup official Kim You-Kyung, Yonhap said the outage was caused by an insider attack, which destroyed computer servers and deleted information on some transaction histories.

"The latest incident was conducted internally...the meticulously designed commands entered through a laptop computer owned by a subcontractor company were carried out to simultaneously destroy the entire server system," said Kim.

AFP added the bank was now unable to bill some of its 5.4 million credit card customers, or settle payments to retailers.

Some 310,000 customers have filed complaints and nearly 1,000 called for compensation from Nonghyup, which has some 5, 000 branches, it said.

Yonhap noted the latest cyberattack comes on the heels of another incident at financial firm Hyundai Capital, where personal information of 420,000 customers were leaked following a systems hack last month.

In a separate report Monday, the Korean news agency said Seoul police sought an arrest warrant for one of the masterminds of the data theft. The 40-year-old man, along with two others, allegedly recruited a famous Korean hacker to break into Hyundai Capital's system.

South Korean police are now seeking Interpol's help to arrest the three masterminds and the hacker, who is said to have fled to the Philippines in 2007. He is wanted for committing several hacking crimes in South Korea.

The financial firm owned by Hyundai Motor Group alerted the police about the data leak on Apr. 7, when it received a blackmail e-mail message demanding money in exchange for the stolen personal data.